I bought my first Gilera (RC 600 -89) last winter. I noticed that first rule is: change the timing belt when you get your Gilera !!! and so I did. After that I drove about 600 km without major problems. Then one afternoon after work I tried to start the bike and I heard some metallic noise from valve train and the engine would not start. For some reason cam belt had jumped and the intake valves had hit the piston and bent. I thought that belt tension was according to the instructions:

TIMING BELT TENSION
In order to carried out this operation, remove the cover located on cylinder head left side. Loosen the belt
tensioning nut (A) and, by means of kick lever, rotate the belt until the piston is at TDC point (compression position
- fully closed valves).
The spring fixed on both sides (B - C pins), will automatically arrange for the belt charging, restoring -the correct
tension. Tighten the nut (A) to the appropriate torque (28 - 30 Kgm).
Warning: Never rotate the timing pulleys acting on the relevant fixing screws.

I didn't find any major faults with the belt. Only one teeth was slightly worn, some metal fibers are visible.

I have now changed the intake valves and cam belt. I started the engine once just to check that everything is ok (no belt jumping this time )

Now I would like to clarify the reason so that this does not happen again . Any ideas what could cause this kind of fault or what I have possibly done wrong? Is there any additional ways to make sure that cam belt tension is correct?

Have you had a look at the backside of the belt?
Belt faults are sometimes caused by a seized tension pulley or a broken water pump.
So maybe you should check both.
The tension pulley has a roller (two) bearing inside which can be replaced.
I think beside this the tension mechanic is a reliable part.
As long as nobody has replaced the spring or mounted the washers in a wrong way.
The water pump should be turned by hand in a smooth way. No heavy friction and no slack.

At the moment it looks like most of teh Gileristi here are watching adult men in shorts, running around on a gras field, hunting a little ball
So maybe the 'real specialists' will answer later.

I said before that I didn't find any major faults with the belt. When I took a closer look to belts I found something. Cam belt that was installed when I bought the bike was Dayco 137 rxr 180. I changed the cam belt because I noticed that first rule is: change the timing belt when you get your Gilera! I got also an unused same type of cam belt with the bike. I didn't installed it because it was not known how old it is. So I ordered a new belt, type is this time Dayco Isoran 321406/2. That was installed when belt slipped and the intake valves hit the piston and bent, only 600 km after replacement. Now I have a brand new same type of belt (321406/2) installed. Engine has been running about 10 minutes and there are already significant signs of wear.

If I can find a brand new Dayco 137 rxr 180, I could check if it wears too.

Comments are very welcome. Replacement of new valves was quite time consuming job so I have to check everything possible so that belt won't slip again. I will not ride with my bike before this problem is solved. Thanks in advance!

Hi
I am not a belt specialist but have looked at your pictures and I don't think that belt 3 and 4 looks correct. The surface of the belt looks too rough and not smooth enough compared to belt 1 and 2. I have also looked at all my 5-6 used belts and they look exactly like 1 and 2. I would defently not install a belt like number 3 and 4. It could look like there have been something wrong in the manufacturing process whith the injecting moulding filling of the rubber material into the tool. Could be too low temperature, too low pressure or other faults.

When mounting a new belt I always increase tension by helping the spring with another 10-20 newton (1-2kg force) by hand only. I think that the spring itself gives a belt tension that is too low.

And of course you have to be sure that the tension wheel is properly locked when the nut is correctly tightend (all washers and distance pieces correct)

And you should check that cam belt tension is abouth the same when you rotate the engine at least 2 revolutions by hand. One of your belt wheels may be out of concentricity. After that, rotate engine by electical starter for 15-30 seconds and recheck tension.

I agree that there is something wrong with belts 3 and 4. Unfortunately I don't have photos of the belts 3 and 4 before installation, it would be interesting to compare those with the used ones. I will get a new cam belt (Dayco 137 RXR 180) next week. Fortunately finnish Gilera expert Jori has those in stock .

Actually I increased the tension with the belt number 4, however with that kind of belt it presumably doesn't solve the problem.

I checked the tension after belt 3 slipped, tension wheel didn't move when I loosened the nut. Tension force was what the spring gives and belt didn't appear to be loose in any way.

Jori looked at the photos and he thinks that the cam wheels are not worn. If wheels were worn, there should a visible ridge. That could happen in hard conditions (dirt , dust etc.) and high mileage. My bike has a quite low mileage (28 000 km) and overall condition shows that it hasn't been driven in extreme conditions.

there is defintly something wrong. A belt should not look like this when it was running a few minutes.
Where did you buy your belt?
I'm just wondering that you have a belt marked with the original Gilera number.
I did not see such a belt since years.
I did not know, but i thought the belts where now all marked with the Dayco number.
Maybe someone else (Joerg?) could confirm this or not.
So maybe you got an 10 years old belt or so.
That yould be a reason for such an excesive wear.

Belts 3 and 4 are from Bob Wright Motorcycles. First one was ordered in February 2014 and the second one in June. When I ordered I told bike type and model, engine serial number and frame serial number so I expected to get the right kind of belt.

I asked Bob if those belts are certainly suitable for my bike and his answer was:

The belts we sell only comes from Piaggio part number 321406 I think Dayco only Make them for Piaggio .You cannot get them else were. I don’t know if your toothed wheels are worn and its showing up on the new belt.

Code is 321406/2. Suffix /2 is version? It means that something has changed from the original belt? Could it be that this belt is suitable for newer Gileras? This is just a wild guess...Has somebody belt 321406/2 in 1989 RC 600, any problems?

in those days when our Gileras where new, Pirelli has been the manufacturer of the belts.
Later they sold the belt business to Dayco.They put the /2 behind the Gilera part number.
The belts are all the same for the 'modern' 4stroke bialberos.
The belts are also available direct from Dayco (not as a Gilera spare part), but not for 'endusers' like we are.
Of course those belts did not have the Gilera, but the Dayco number.

I also don't have any doubt about Bob Wright not being a reliable supplier. I have bought other parts from him and all have been top quality. I told Bob that I will ask for help from forum Gileraclub.de and sent link to this discussion.

After I'll get a new belt Dayco 137RXR180, we will hopefully know more. Is the problem in my bike, installation of the belt or is there something wrong with 321406/2 belts. If the problem is in the belt maybe Bob could check that with Piaggio.

Made in Italy?
Maybe this one is a little bit older
Since years mine did not come from Italy any more.
But also i did not buy them from Gilera but from Gottfried, or nox from another forum member.
Hope that you have now found the fault and could enjoy your RC